1. Field of the Invention
This invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a semiconductor device including a bipolar transistor having a shallowed base whose width is reduced and a method for manufacturing the same.
2. Description of the Related Art
The high-frequency characteristic of a bipolar transistor depends on the base width.
In the conventional technique, the intrinsic base region is formed by implanting impurities such as boron (B) or boron fluoride (BF.sub.2) into a silicon substrate. When the base junction is set in a shallow position by use of the above technique, the ion implantation energy is lowered. By lowering the implantation energy, the impurity injection distance from the surface of the substrate can be made short and a base with a shallow base junction can be formed.
However, the above ion implantation technique has a problem of channeling phenomenon. The channeling phenomenon is that impurities are implanted into a position which is deeper than the implantation depth theoretically estimated from the physical energy of the implanted impurities. Such a phenomenon becomes significant when impurities such as boron having low atomic weight are used. When this phenomenon occurs, it becomes possible to make the junction position shallow even if the implantation energy is lowered to the lowest possible level. Further, the upper limit of the implantation energy itself in the manufacturing process is set to 10 KeV when taking the ion beam current into consideration. This means that the improvement of the performance of the bipolar transistor which has been attained by making the base junction shallow (or shallowing the base junction) is approaching its limitation.